(1) Field of the Invention
The present invention relates to a mercury recovery method and a mercury recovery apparatus, and particularly to a technique for separating and collecting mercury contained in phosphors used in fluorescent lamps.
(2) Related Art
Mercury, which is a toxic material, is used in lighting devices such as fluorescent lamps. When fluorescent lamps whose life is expired or fluorescent lamps that are found defective during manufacturing processes (hereafter simply referred to as xe2x80x9cwaste fluorescent lampsxe2x80x9d) are disposed, mercury is usually to be recovered from the waste fluorescent lamps. The recovered mercury is processed to remove impurities etc., and then is recycled.
As a method for recovering mercury from phosphor powder used in waste fluorescent lamps, the wet-method is conventionally known, one example of which is disclosed in Japanese laid-open patent application No.H10-12149. According to the disclosed method, mercury-containing phosphor powder detached from a waste fluorescent lamp is immersed in an aqueous solution in which active sulfur is dissolved, to form a mercuric sulfide compound that is refractory, so that mercury can be recovered from the phosphor powder.
Also, as another method for recovering mercury from phosphor powder, the dry-method is conventionally known, one example of which is disclosed in Japanese published examined patent application No.S53-1594. This method takes advantage of a characteristic of mercury as being a low-boiling metallic element. According to the disclosed method, mercury-containing phosphor powder is subjected to a heating process to vaporize mercury contained therein, and the vaporized mercury is then cooled and condensed, so that mercury can be recovered from the phosphor powder.
Here, rare-earth phosphors, which are expensive, are often used in fluorescent lamps for general lighting. When waste fluorescent lamps are disposed, therefore, it is desirable to recycle not only mercury, but also phosphor powder from which mercury is separated (hereafter simply referred to as xe2x80x9ctreated phosphor powderxe2x80x9d).
However, with the above-described conventional wet-method, a portion of the phosphor powder is dissolved into the aqueous solution containing active sulfur, with the crystal structure of phosphors being destroyed or changed. This causes characteristics of the phosphor powder including luminance to deteriorate, making recycling of the treated phosphor powder impossible.
Further, unlike the dry-method mercury recovery, the wet-method mercury recovery generally requires special equipment for liquid-waste treatment, including treatment of used aqueous solutions etc. Therefore, the wet-method mercury recovery tends to suffer from high-cost, compared with the dry-method mercury recovery.
With the conventional dry-method mercury recovery, substantially all mercury, when being contained in the form of metallic mercury or mercurous oxide, can be separated and collected from phosphor powder. However, the problem is that phosphor powder of life-expired fluorescent lamps, in particular, contain mercury in the form of an amalgam that is formed by reacting with an emitter (emissive material) or a phosphor element. It is difficult to decompose an amalgam by heat, and accordingly it is difficult to separate and collect substantially all mercury from such phosphor powder. To solve this problem, increasing a heating temperature in the heating process may be considered. However, the temperature being too high in the heating process causes thermal degradation of phosphor powder, which results in characteristics of the phosphor powder including luminance deteriorating, making recycling of the treated phosphor powder impossible.
In view of the above problems, a first objective of the present invention is to provide a mercury recovery method that ensures recovery of mercury in any forms, such as metallic mercury, from phosphor powder, and that enables the treated phosphor powder to be recycled, without requiring special equipment for liquid-waste treatment. A second objective of the present invention is to provide a mercury recovery apparatus that produces the same effect as the mercury recovery method.
The first objective of the present invention can be achieved by a mercury recovery method for recovering mercury from mercury-containing phosphor powder that has been detached from a fluorescent lamp, the mercury recovery method including the steps of: vaporizing mercury contained in the phosphor powder by subjecting the phosphor powder to a heating and reducing process; and condensing the vaporized mercury by cooling, to collect the vaporized mercury.
According to this method, a heating process is carried out in a reducing atmosphere. Therefore, mercury contained in phosphor powder in any forms, such as metallic mercury, mercurous oxide, and an amalgam formed by reacting with an emitter or a phosphor element, can be reduced in the heating process. Therefore, atomization of the mercury in any forms can be facilitated. This can ensure vaporization and separation of substantially all mercury contained in the phosphor powder even at relatively low temperatures. In particular, because the mercury can be vaporized and separated at relatively low temperatures, thermal degradation of the phosphor powder can be reduced. As a result, characteristics of the phosphor powder including luminance can be prevented from deteriorating, enabling the treated phosphor powder to be recycled. Further, because the mercury recovery is achieved with the dry-method, special equipment for liquid-waste treatment that is usually required by the wet-method mercury recovery is not required.
The second objective of the present invention can be achieved by a mercury recovery apparatus that recovers mercury from mercury-containing phosphor powder, including: a reaction vessel; a distillation vessel that is set in the reaction vessel and in which the mercury-containing phosphor powder is placed; a heating unit for heating the distillation vessel; and a mercury collection unit for condensing vaporized mercury by cooling, to collect the vaporized mercury, the vaporized mercury having been vaporized from the mercury-containing phosphor powder in the distillation vessel heated by the heating unit, wherein the distillation vessel includes a heat conductive member that is placed so as to come in contact with a part of an interior wall of the distillation vessel, the heat conductive member being for transmitting heat to middle portions of the phosphor powder within the distillation vessel.
According to this construction, the entire portions of phosphor powder placed in the distillation vessel can be heated uniformly. This can facilitate vaporization and separation of mercury from the phosphor powder. Therefore, the temperature of middle portions of the phosphor powder in the distillation vessel can be increased to a temperature necessary for thermal decomposition of mercury compounds, without excessively increasing a heating temperature by the heating unit. Also, because the phosphor powder is not heated up to excessively high temperatures, thermal degradation of the phosphor power can be reduced. As a result, characteristics of the phosphor powder including luminance can be prevented from deteriorating, enabling the treated phosphor powder to be recycled. Also, because the mercury recovery is achieved with the dry-method, special equipment for liquid-waste treatment that is usually required by the wet-method mercury recovery is not required.